1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and more particularly relates to a twisted-nematic (TN) LCD for conducting a display operation in Normally Black mode. The present invention also relates to a multilayer phase plate for use in such an LCD.
2. Description of the Related Art
TN LCDs for conducting a display operation in Normally White mode are currently used extensively.
In a Normally White mode LCD, a portion of each pixel, which is not covered with its associated pixel electrode (e.g., a gap between a bus line and the pixel electrode), is always supplied with no voltage and always transmits light. This light causes no problem in white display state but becomes unintentionally leaking light in black display state, thus decreasing the contrast ratio of the display screen. In view of this potential problem, the front substrate normally includes an opaque layer to minimize such leakage of light.
In order to increase an allowable margin in bonding the two substrates of an LCD together, the area of such an opaque layer is normally set broader than the total area of the light leaking regions, thus decreasing the aperture ratio of pixels significantly. Accordingly, such an opaque layer constitutes a major obstacle to realizing an LCD with a sufficiently high aperture ratio.
Thus, to overcome such a problem, a normally black mode LCD for carrying out a black display operation with no voltage applied has been proposed. A normally black mode TN LCD performs a black display operation with liquid crystal molecules oriented in a twisted pattern in the thickness direction of a liquid crystal layer.
In the normally black mode TN LCD, however, the refractive index anisotropy of its liquid crystal layer usually has wavelength dependence (or wavelength dispersion characteristic), thus posing another problem of colored black display.
Hereinafter, it will be described how this problem happens on a liquid crystal layer, of which the retardation Δn·d is controlled so as to set its first minimum condition at 550 nm. If a linearly polarized light ray with a wavelength of 550 nm is allowed to be incident on this liquid crystal layer in the black display state (i.e., a state in which the liquid crystal molecules are oriented at a twist angle of 90 degrees), then the incoming linearly polarized light ray will have its polarization direction rotated by 90 degrees when leaving the liquid crystal layer. That is to say, the polarization direction of the outgoing linearly polarized light ray will be perpendicular to that of the incoming linearly polarized light ray. However, since the refractive index anisotropy of the liquid crystal layer has wavelength dispersion characteristic, an incoming linearly polarized light ray with any other wavelength will be an elliptically polarized light ray when leaving the liquid crystal layer. Furthermore, the major-axis direction of such an elliptically polarized light ray is not aligned with the polarization direction of the outgoing linearly polarized light ray with the wavelength of 550 nm. Such misalignment of the major-axis direction is unique to a liquid crystal layer with the twisted orientation and is never observed in any liquid crystal layer with non-twisted homogeneous orientation.
In this manner, in the normally black mode TN LCD, light that has been transmitted through its liquid crystal layer in the black display state includes not only the desired linearly polarized light ray but also other components as well. The linearly polarized light ray with the wavelength of 550 nm is blocked by the polarizer located closer to the viewer of the LCD but light rays with other wavelengths are transmitted through the same polarizer to a certain degree. As a result, the black display is somewhat colored and the contrast ratio decreases.
To overcome such a problem, a method of doing color compensation with a lot of phase films was proposed by Sergan and three others in “Achromatic Normally Black Twisted Nematic Device with Wide Viewing Angle Using Negative-In-Plane Compensation Films”, Jpn. J. Appli. Phys., March, 1998, Vol. 37, pp. 889-894. A method of color compensation using a twisted phase plate, of which the slow axis is twisted in the thickness direction and which has wavelength dependence opposite to that of the liquid crystal layer, was also proposed for the same purpose in Japanese Laid-Open Publication No. 2001-188232, for example.
However, the former technique uses a lot of phase films, thus raising the manufacturing cost and the thickness of the display device. According to the latter technique on the other hand, the productivity of the twisted phase plate is too low to use it industrially, thus increasing the manufacturing cost, too.